Rockets launched from beneath the water displace water as the rocket passes upward. Since the rocket is slowing as it rises, the water, which surges into the space behind the passing rocket can achieve a speed greater than the upward moving rocket and strike the rocket nozzle with a jet as the rocket leaves the water. This jet can interact with the rocket exhaust at ignition and damage the nozzle, the nozzle mounts, or nozzle motion actuators.
The launch environment uses a gas pressure considerably above atmospheric pressure to thrust the rocket through the water toward the surface. A gas tight seal over the nozzle such as shown in U.S. Pat. No. 4,949,618 prevents the launch gases from equalizing pressure inside the rocket and endangers the integrity of the rocket. Any protection system must permit the flow of gas while protecting the rocket against a water jet.